1. Technical Field
The present invention relates generally to a steering control system for a vehicle. More particularly, the invention relates to an improved rear wheel steering system for a vehicle which is adapted for detecting slip of driven wheels to determine a rear wheel sterring angle based thereon.
2. Background Art
It is well known in the art that four-wheel steered (4WS) vehicles are adapted for providing cornering forces on rear wheels during turns without slipping to improve turning stability.
A Japanese Patent First Publication No. 57-60974 discloses a rear wheel steering system. This system is operable to detect acceleration or centripetal force acting on a vehicle body during turns to steer rear wheels based thereon to the same phase as a steered angle of front wheels in the case of a front-wheel drive (FWD) vehicle or to a phase opposite the front wheels in the case of a rear-wheel drive (RWD) vehicle for preventing a spin of the FWD vehicle which is caused by driven wheel slippage during turning with acceleration or, in the case of rear-wheel drive vehicles, for preventing drift-out.
The above prior art steering system is however provided with an acceleration sensor which is installed on a part of the vehicle body to detect acceleration including an unnecessary component induced by vehicle motion such as vertical displacement or pitching due to uneven road or acceleration, resulting in unsuitable rear wheel steering control.
For avoiding such drawbacks, a Japanese Patent First Publication No. 62-71761 discloses a rear wheel steering system which controls a steering operation for rear wheels based on a difference in rotational speeds between driven and compliance wheels to prevent spin or drift-out due to driven wheel slippage (i.e., due to rotational speed of the driven wheels being higher than that of the compliance wheels) during turning with acceleration.
However, the prior art rear wheel steering control system is responsive to a difference in rotational speeds between the driven and compliance wheels during turns to correct a steering ratio (i.e., a ratio of the steering angle of the rear wheels to the front wheels) based thereon and controls rear wheel steering according to the corrected steering angle ratio. Thus, this system has a drawback in that there is a tendency for the rear wheel steering control to become unstable when a brake pedal is depressed during turning.
Braking forces of the front and rear wheels are normally not constant when a braking occurs and any of the wheels may sometimes lock early. This results in a difference in rotational speeds between the driven and compliance wheels due to the wheel locking (hereinafter, this difference in rotational speeds due to braking is referred to as braking slips). As a result of braking slips in addition to driven wheel slip being detected as a rotational speed difference between the driven and compliance wheels, a steering ratio set according to such a detected value becomes unsuitable, further reducing steering stability during turns.